In an important industrial process for producing pentaerythritol, formaldehyde is reacted with acetaldehyde in the presence of metal hydroxide catalyst, and the main bulk of pentaerythritol product is recovered from the aqueous reaction medium by crystallization. Typical pentaerythritol processes are described in U.S. Pat. Nos. 2,790,836 and 3,968,176.
The aqueous mother liquor resulting from the manufacture of pentaerythritol is a waste byproduct stream containing metal formate and unrecovered pentaerythritol, and soluble organic byproduct materials formed in the pentaerythritol synthesizing reaction. This stream represents a difficult waste disposal problem, even though it contains metal formate and pentaerythritol of substantial commercial value. If it is attempted to recover the metal formate by conventional evaporative crystallization means, the presence of the water-soluble organic materials interferes with and impedes this recovery. Because the organic materials become concentrated in the crystallization mother liquor, it becomes quite viscous, thereby hampering crystal separation from the liquor and causing contamination of the separated crystals. In practice, the recovery of metal formate from the liquor is limited to only 60-75% of the total metal formate present, owing to these factors. As a result, a substantial quantity of viscous organic-rich material is produced as a byproduct, which still poses a formidable waste disposal problem.
The potential for recovery of valuable byproducts contained in the aqueous waste stream of a pentaerythritol manufacturing operation has invited various developments which have been reported in the prior art literature.
British Pat. No. 848,632 (1960) describes a method for recovering pentaerythritol and sodium or calcium formate from a pentaerythritol aqueous waste stream which involves evaporating the waste stream to remove about 70% of the water, diluting the concentrate with methanol, removing formate solids, cooling the filtrate to 15.degree. C. and recovering pentaerythritol crystals.
U.S. Pat. No. 2,441,602 describes a pentaerythritol aqueous waste stream treatment method which involves concentrating the stream by water removal, diluting the concentrated stream with a water-soluble monohydric alcohol and heating the diluted mixture, then separating the undissolved metal formate by filtration, distilling the filtrate solution to remove most of the water as an azeotrope with the alcohol, and subsequently cooling the solution to produce crystallized metal formate and pentaerythritol.
U.S. Pat. No. 2,617,791 describes a process for treating pentaerythritol mother liquor which involves heating the mother liquor with a fatty acid at 175.degree.-275.degree. C. until an oily phase and a solids phase are formed, and thereafter recovering the oily layer which contains fatty acid esters of the polyhydroxy compounds.
U.S. Pat. No. 2,780,655 describes a method of treating pentaerythritol mother liquor which involves concentrating the mother liquor to form a slurry of solid pentaerythritol and metal salts, adding formalin to the slurry to dissolve the pentaerythritol, and recovering the undissolved metal salts.
U.S. Pat. No. 3,179,704 describes a method of treating pentaerythritol mother liquor which involves evaporating the mother liquor to dryness, admixing the resulting dried solids with dimethylformamide to dissolve the organic materials, and separating the extract solution from the undissolved formate salts.
U.S. Pat. No. 3,766,277 describes a method of treating an aqueous solution containing pentaerythritol and metal alkanoate which involves contacting the solution with tertiary-butyl alcohol to extract pentaerythritol, and thereafter recovering pentaerythritol by crystallization from the solvent phase.
Other United States patents of general interest relating to the production and recovery of pentaerythritol and various byproducts include U.S. Pat Nos. 2,004,010; 2,223,421; 2,270,839; 2,386,289; 2,533,737; 2,696,507; 2,719,867; 2,782,918; 2,790,011; 2,790,836; 2,790,837; 2,820,066; 3,379,624; 3,478,115; 3,875,248; 3,968,176; 4,083,931; 4,105,575; and references cited therein.
Because of environmental and economic considerations, as indicated by the prior art references disclosed above, there has been continuing investigative effort to develop methods for recovering the valuable organic and inorganic components of waste byproduct streams such as that associated with pentaerythritol production.
Accordingly, it is a main object of the present invention to provide an improved process for recovering organic and inorganic values from the aqueous waste stream derived from pentaerythritol manufacture.
It is another object of this invention to provide a method of recovering in high purity form substantially all of the metal formate contained in an aqueous waste stream derived from pentaerythritol production, in which production formaldehyde is reacted with acetaldehyde in the presence of a metal hydroxide catalyst.
It is another object of this invention to provide a method for recovering substantially all of the sodium formate and residual pentaerythritol contained in an aqueous waste stream derived from pentaerythritol production, in which production formaldehyde is reacted with acetaldehyde in the presence of a sodium hydroxide catalyst.
It is a further object of this invention to provide a method for recovering byproduct values from a pentaerythritol production waste stream, wherein the method yields a residual mother liquor concentrate which has high fuel value and improved burning characteristics.
Other objects and advantages of the present invention shall become apparent from the accompanying description and example.